The present invention generally relates to a system and method of clearance control of motor or engine fan blades, and more particularly relates to a system and method of determining thermal growth of motor or engine parts to thereupon control the clearance of motor or engine fan blades.
The knowledge and control of radial growth of turbo-machinery components has long been a stumbling block on the way to achieving higher efficiency and stability levels demanded by the designers of gas turbine engines, pumps and compressors. This undesirable situation is driven in part by lack of reliable, accurate and affordable sensors for measuring radial growth. Alternatively, the radial growth can be computed using a mathematical model that relates growth to various turbo-machine measured and otherwise obtained parameters. Numerous attempts were made in the past to devise such an algorithm. However, none of the known algorithms delivered required steady state and transient accuracy, ability to calibrate the equations to high fidelity data and formulation suitable for implementation in a digital computer.
Imperfect control of the clearance between a turbine engine fan blade and case results in either the clearance being too loose or the clearance being too tight resulting in excessive rubs. In either instance, imperfect clearance results in loss of performance (e.g. engine efficiency, thrust) and/or violation of the engine operating limits (e.g. exhaust gas temperature overshoot) and/or reduced compressor stability. Standard practice has been to design a clearance control system to prefer loose clearance over fight clearance which may also result in damage to the blades and case. Some engines such as, for example, the PW4000 use an open loop clearance control system that sacrifices significant performance in comparison with a xe2x80x9cperfectxe2x80x9d clearance control system. Other engines such as, for example, the V2500 use a closed loop system that relies on crudely modeled clearances and therefore sacrifices less performance, but still falls short of ideal clearance control.
Improved accuracy and reliability in estimating tip clearances will also enable the clearance control system to be active during those parts of an airplane mission that are more likely to experience abrupt changes in operating conditions. For example, a typical active clearance control system is traditionally deactivated during airplane takeoff where tip clearances are particularly hard to predict due to rapidly changing engine operating conditions. This approach worked well in the past for the cases where takeoff constituted a relatively small portion of the overall airplane mission and the engine stability margins were conservatively high. In contrast, takeoff fuel economy gains importance for the engines designed for short haul aircraft applications such as, for example, PW6000 designed for A318 application. The ability to deploy active clearance control during takeoff also increases the exhaust gas temperature margin which otherwise diminishes with increased clearance, and helps to avoid clearance induced stability loss. Thus, it is desirable to further improve clearance control accuracy to, in turn, improve engine performance while maintaining all operating limits, compressor stability and ensuring reliable rub-free operation throughout the airplane mission
The principal difficulty in modeling clearances for a closed loop system resides in modeling the thermal growths of the engine components, not in modeling the mechanical strains which are relatively easy to calculate. Thermal growths are far more difficult to model because the physical configurations of the engine components and the multiple time varying influences to which those components are subjected (i.e., throttle transients, multiple fluid streams of different and time varying temperatures, flow rates, etc.) complicate the problem of modeling the heat transfer and energy storage phenomenon.
In view of the foregoing, it is a general object of the present invention to provide a system and method of clearance control that overcomes the above-mentioned drawbacks.
In one aspect of the present invention a method of controlling clearance in a turbomachine between blades and a wall adjacent to and opposing tips of the blades includes adjusting air flow adjacent to the wall in response to the difference between the desired clearance and the actual clearance. Due to lack of suitable sensors, an accurate and reliable estimate of the actual clearance is made with a real time, mathematical model running, on-board engine controller. As part of the clearance calculation the subject model computes thermal growth of the turbomachine components and their subcomponents with a difference equation derived from a closed form solution to the 1st order differential equation obtained through the application of the 1st law of thermodynamics. The component is treated as being made of uniform material with given average specific heat and mass while at uniform temperature throughout the volume. The heat transfer phenomenon is modeled as a sum of a finite number of heat transfer processes taking place over the entire area of the component interface with the known gas turbine fluid streams. Each heat transfer process is characterized by a local average heat transfer coefficient, contact surface area and fluid temperature. The solution is defined over a time step of the control software assuming either step or ramp input and is expressed in terms of equivalent time constant, steady state growth and a set of approximating coefficients. The steady state growth is calculated as a weighted average of the growths due to thermal exchange with fluid streams of varying temperatures, flow rates and thermo-physical properties. The weighting is accomplished with performance parameters that are first formed as functions of the local fluid stream area of contact, the local heat transfer coefficient, the total component mass and the component average specific heat. Then, recognizing that these characteristics are impractical to define for a specific component or its subcomponents, the performance parameters are correlated with the measured and otherwise synthesized engine characteristics such as shaft speeds, pressures and temperatures. Finally, the inverse of the equivalent time constant is calculated as a sum of the same performance parameters.
In a second aspect of the present invention, a system for controlling clearance in a turbomachine between blades and a wall adjacent to and opposing tips of the blades includes adjusting air flow adjacent to the wall in response to the difference between the desired clearance and the actual clearance. Due to lack of suitable sensors, an accurate and reliable estimate of the actual clearance is made with a real time, mathematical model running, on-board engine controller. As part of the clearance calculation the subject model computes thermal growth of the turbomachine components with a difference equation derived from a closed form solution to the 1st order differential equation obtained through the application of the 1st law of thermodynamics. The component is treated as being made of uniform material with given average specific heat and mass while at uniform temperature throughout the volume. The heat transfer phenomenon is modeled as a sum of a finite number of heat transfer processes taking place over the entire area of the component interface with the known gas turbine fluid streams. Each heat transfer process is characterized by a local average heat transfer coefficient, contact surface area and fluid temperature. The solution is defined over a time step of the control software assuming either step or ramp input and is expressed in terms of equivalent time constant, steady state growth and a set of approximating coefficients. The steady state growth is calculated as a weighted average of the growths due to thermal exchange with fluid streams of varying temperatures, flow rates and thermo-physical properties. The weighting is accomplished with performance parameters that are first formed as functions of the local fluid stream area of contact, the local heat transfer coefficient, the total component mass and the component average specific heat. Then, recognizing that these characteristics are impractical to define for a specific component, the performance parameters are correlated with the measured and otherwise synthesized engine characteristics such as shaft speeds, pressures and temperatures. Finally, the inverse of the equivalent time constant is calculated as a sum of the same performance parameters.
In a third aspect, a gas turbine engine system comprises an engine including case and disk with blades rotatable within the case. Means are provided for controlling clearance in a turbomachine between blades and a wall adjacent to and opposing tips of the blades to adjust air flow adjacent to the wall in response to the difference between the desired clearance and the actual clearance. Due to lack of suitable sensors, an accurate and reliable estimate of the actual clearance is made with a real time, mathematical model running, on-board engine controller. As part of the clearance calculation the subject model computes thermal growth of the turbomachine components with a difference equation derived from a closed form solution to the 1st order differential equation obtained through the application of the 1st law of thermodynamics. The component is treated as being made of uniform material with given average specific heat and mass while at uniform temperature throughout the volume. The heat transfer phenomenon is modeled as a sum of finite number of heat transfer processes taking place over the entire area of the component interface with the known gas turbine fluid streams. Each heat transfer process is characterized by a local average heat transfer coefficient, contact surface area and fluid temperature. The solution is defined over a time step of the control software assuming either step or ramp input and is expressed in terms of equivalent time constant, steady state growth and a set of approximating coefficients. The steady state growth is calculated as a weighted average of the growths due to thermal exchange with fluid streams of varying temperatures, flow rates and thermo-physical properties. The weighting is accomplished with performance parameters that are first formed as functions of the local fluid stream area of contact, the local heat transfer coefficient, the total component mass and the component average specific heat. Then, recognizing that these characteristics are impractical to define for a specific component, the performance parameters are correlated with the measured and otherwise synthesized engine characteristics such as shaft speeds, pressures and temperatures. Finally, the inverse of the equivalent time constant is calculated as a sum of the same performance parameters.
An advantage of the present invention is that the thermal growths of turbomachine components and its sub-components may be accurately and reliably determined for subsequent use in the estimation of actual clearance of a turbomachine which is a required element in closed loop active clearance control schemes such as one typically employed in a turbine case cooling.
A second advantage of the present invention is that an accurate numerical technique, namely a closed form solution to the governing differential equation applied over time step of control software is employed to implement 1st order dynamics of the thermal growth.
A third advantage is that the effect of multiple fluid streams is accounted for in the calculation of both steady state and transient characteristics of the thermal growth.
A fourth advantage is the allowance of finite order dynamics to be implemented as the sum of 1st order dynamic elements. Thus, to model a component of complex geometry and heat transfer phenomenon the component can be subdivided into a finite number of sub-components each modeled with a 1st order dynamic element.
A fifth advantage is the use of approximating coefficients to allow for calibration against high fidelity models and test data, and to facilitate the model traceability to a specific set of turbomachine hardware which may be changing through the operational life of the machine.
A sixth advantage is that the physics-based model increases chances of sensible results outside the machine operating envelope.